Multiple-core planar optical waveguides and methods of fabrication and use thereof

1. An optical waveguide, comprising:

a substantially planar waveguide substrate;

a lower waveguide core;

an upper waveguide core, at least a portion the upper waveguide core being positioned above and substantially parallel to at least a portion of the lower waveguide core;

lower cladding between the substrate and the lower waveguide core having a refractive index less than refractive indices of the lower and upper waveguide cores; and

upper cladding above the upper waveguide core having a refractive index less than refractive indices of the lower and upper waveguide cores,

wherein:

a width of the lower waveguide core is substantially larger than a thickness thereof along at least a portion thereof, and is substantially flat along that portion of the length thereof, thereby yielding a substantially flat surface for forming at least a portion of the upper waveguide core;

the upper and lower waveguide cores are in contact with one another over at least a portion of their respective lengths, and the flat surface comprises a portion of an upper surface of the lower waveguide core;

one of the upper and lower waveguide cores comprises a low-index-contrast core, and the other of the upper and lower waveguide cores comprises a high-index-contrast core;

the high-index-contrast waveguide core tapers in at least one transverse dimension in a longitudinal direction along the flat portion of the lower waveguide core;

the tapered portion of the high-index-contrast core and the portion of the low-index-contrast core in contact with the high-index-contrast core are arranged so as to guide a selected propagating optical mode having a transverse spatial profile that evolves along the length of the tapered portion of the high-index-contrast core;

the high-index-contrast waveguide core tapers sufficiently gradually so as to substantially avoid optical coupling between the selected propagating optical mode and other optical modes;

the high-index-contrast core comprises silicon nitride or silicon oxynitride less than about 200 nm thick and less than about 2 μm wide along a non-tapered portion thereof;

the low-index-contrast core comprises silica or doped silica between about 0.4 μm thick and about 1 μm thick and between about 4 μm wide and about 10 μm wide along at least the flat portion of the lower waveguide; and

the upper and lower cladding comprise silica or doped silica.

2. An optical waveguide, comprising:

a substantially planar waveguide substrate;

a lower cladding layer formed on the substrate;

a lower waveguide core formed on the lower cladding layer;

an upper waveguide core formed above the lower cladding layer, at least a portion of the upper waveguide core being positioned above, substantially parallel to, and in contact with at least a portion of the lower waveguide core; and

an upper cladding layer formed over the upper waveguide core and the lower waveguide core,

wherein:

each of the upper and lower cladding layers has a respective refractive index less than refractive indices of the lower and upper waveguide cores;

the lower waveguide core has, at least along the portion in contact with the upper waveguide core, a substantially flat upper surface;

one of the upper and lower waveguide cores comprises a low-index-contrast core, and the other of the upper and lower waveguide cores comprises a high-index-contrast core;

at least a portion of the high-index-contrast core that is in contact with the low-index-contrast core has a substantially constant vertical transverse extent and has a tapered horizontal transverse extent;

the high-index-contrast core comprises silicon nitride or silicon oxynitride less than about 2 μm wide along a non-tapered portion thereof and less than about 200 nm thick;

the low-index-contrast core comprises silica or doped silica between about 0.4 μm thick and about 1 μm thick and between about 4 μm wide and about 10 μm wide along at least the flat portion of the lower waveguide; and

the upper and lower cladding comprise silica or doped silica.

3. A method for forming an optical waveguide, comprising:

forming a lower cladding layer on a waveguide substrate;

forming a lower waveguide core on the lower cladding layer;

forming an upper waveguide core above the lower cladding layer, at least a portion of the upper waveguide core being positioned above, substantially parallel to, and in contact with at least a portion of the lower waveguide core; and forming an upper cladding layer over the upper waveguide core and the lower waveguide core,

wherein:

each of the upper and lower cladding layers has a respective refractive index less than refractive indices of the lower and upper waveguide cores;

the lower waveguide core has, at least along the portion in contact with the upper waveguide core, a substantially flat upper surface;

one of the upper and lower waveguide cores comprises a low-index-contrast core, and the other of the upper and lower waveguide cores comprises a high-index-contrast core;

at least a portion of the high-index-contrast core that is in contact with the low-index-contrast core has a substantially constant vertical transverse extent and has a tapered horizontal transverse extent;

the high-index-contrast core comprises silicon nitride or silicon oxynitride less than about 2 μm wide along a non-tapered portion thereof and less than about 200 nm thick;

the low-index-contrast core comprises silica or doped silica between about 0.4 μm thick and about 1 μm thick and between about 4 μm wide and about 10 μm wide along at least the flat portion of the lower waveguide; and

the upper and lower cladding comprise silica or doped silica.